Fluidity of Structure and Swiveling of Helices in the Subunit c Ring of Escherichia coli ATP Synthase as Revealed by Cysteine-Cysteine Cross-Linking

Department of Biochemistry, University of Wisconsin–Madison, Madison, Wisconsin, United States
Journal of Biological Chemistry (Impact Factor: 4.57). 12/2007; 282(46):33788-94. DOI: 10.1074/jbc.M706904200
Source: PubMed


Subunit c in the membrane-traversing F(0) sector of Escherichia coli ATP synthase is known to fold with two transmembrane helices and form an oligomeric ring of 10 or more subunits in the membrane. Models for the E. coli ring structure have been proposed based upon NMR solution structures and intersubunit cross-linking of Cys residues in the membrane. The E. coli models differ from the recent x-ray diffraction structure of the isolated Ilyobacter tartaricus c-ring. Furthermore, key cross-linking results supporting the E. coli model prove to be incompatible with the I. tartaricus structure. To test the applicability of the I. tartaricus model to the E. coli c-ring, we compared the cross-linking of a pair of doubly Cys substituted c-subunits, each of which was compatible with one model but not the other. The key finding of this study is that both A21C/M65C and A21C/I66C doubly substituted c-subunits form high yield oligomeric structures, c(2), c(3)... c(10), via intersubunit disulfide bond formation. The results indicate that helical swiveling, with resultant interconversion of the two conformers predicted by the E. coli and I. tartaricus models, must be occurring over the time course of the cross-linking experiment. In the additional experiments reported here, we tried to ascertain the preferred conformation in the membrane to help define the most likely structural model. We conclude that both structures must be able to form in the membrane, but that the helical swiveling that promotes their interconversion may not be necessary during rotary function.

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    • "An alternative model of the mechanical cycle in F-type ATPases has recently been proposed by Dimroth et al. [9] [10], based on the structure of the rotor ring of the Na + -ATPase from Ilyobacter tartaricus. The main structural feature of this model is the presence of the ion binding glu-65 of c subunits facing towards the outer surface of the rotor ring, which suggests that a conformational change may not be necessary for generating torque for rotation of the F 0 c-ring [3] [8] [11]. Therefore, many questions still remain regarding the coupling between proton transport and rotational motion. "
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